Generally, an electroluminescent element (hereinafter abbreviated as EL element) which emits light upon application of an AC field has a structure in which a filmy layer of a dielectric is provided on one side or both sides of a thin layer of an electroluminescent phosphor and these laminate layers are sandwiched by two electrode layers. The phosphor layer used in such element is basically composed of such material as ZnS, ZnSe or ZnF.sub.2 doped Mn or a rare-earth fluoride as a luminescent center in said base material. A ZnS phosphor element using Mn as a luminescent center is capable of providing a luminance of up to about 3,500-5,000 Cd/m.sup.2 by the application of an AC voltage with a frequency of 5 kHz.
Typical examples of dielectric materials used in said element are Y.sub.2 O.sub.3, SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5. The thickness of the ZnS layer is about 5,000 to 7,000 .ANG. and that of the dielectric layer is about 4,000 to 8,000 .ANG..
In the case of AC drive, the voltage applied to the element is divided to the ZnS layer and the dielectric layer. Since the EL element is structurally equivalent to a series connection of two capacitors, there holds the relation of .epsilon..sub.i V.sub.i /t.sub.i =.epsilon..sub.z V.sub.z /t.sub.z (.epsilon.: dielectric constant; V: voltage applied; t: thickness; suffix i: indicating dielectric; suffix z: indicating ZnS), and thus each divided voltage is reversely proportional to the dielectric constant if t.sub.i =t.sub.z. In said dielectrics such as Y.sub.2 O.sub.3, .epsilon..sub.i is about 4 to 25 and .epsilon..sub.z of ZnS is about 9, so that only about 30 to 70% of the whole applied voltage is given to the ZnS layer. In such elements, therefore, a high voltage above 200 V must be applied by a pulse drive of several kHz. Such high voltage gives a great deal of load to the drive circuit and necessitates a specific high-voltage withstanding drive IC, which leads to the increased production cost of the element.
A discussion is here made on what characteristics the dielectric layer is required to have for reducing the drive voltage. From the above-shown relation concerning voltage division, it is noted that the .epsilon..sub.i to t.sub.i ratio (.epsilon..sub.i /t.sub.i) must be great. After the start of emission of light, any increment of applied voltage is given to the dielectric layer, so that V.sub.ib (dielectric breakdown voltage of the dielectric layer) must be also high for giving an excellent dielectric film. Therefore, the figure of merit .gamma. of the dielectric layer is defined as follows: EQU .gamma.=.epsilon..sub.i V.sub.ib /t.sub.i =.epsilon..sub.i E.sub.ib
(E.sub.ib : dielectric breakdown field strength of the dielectric film)
As noted from the above equation, .gamma. is proportional to the electric charge accumulated per unit area of the dielectric layer when dielectric breakdown occurs. The greater the value of .gamma., the more stably can be conducted the low-voltage drive. This can be attributed to the following fact. In two EL elements which are same in phosphor layer thickness and dielectric layer thickness but different in properties of dielectric layer (for example, the dielectric layer in one of the elements has the properties of .epsilon..sub.i =100, E.sub.ib =1.times.10.sup.6 V/cm and .gamma.=100.times.10.sup.6 V/cm while the dielectric layer in another element has the properties of .epsilon..sub.i =50, E.sub.ib =3.times.10.sup.6 V/cm and .gamma.=150.times.10.sup.6 V/cm), naturally the former element can start to emit at a lower voltage than the latter element as they have the same thickness dielectric layer. However, in the latter element where .epsilon..sub.i =50 and E.sub.ib =3.times.10.sup.6 V/cm, if it is equalized to the former element in breakdown strength, its layer thickness can be reduced to 1/3. Consequently, its dielectric capacity is trebled, boosting .epsilon..sub.1 to 150. Therefore, a greater figure of merit allows the production of an element which emits light at a lower voltage, regardless of .epsilon..sub.i. The greater the value of .gamma., the better, but practically, it is desirable that .gamma. is about 10 times the value of 14.times.10.sup.6 V/cm that is obtained by substituting .epsilon..sub.z =9 and E.sub.zb =1.6.times.10.sup.6 V/cm of ZnS for .epsilon..sub.i and E.sub.ib in the above-shown formula and used as a standard value for low-voltage luminescence.
Conventional dielectric films are small in their figure of merit, which is about 50.times.10.sup.6 V/cm in the case of Y.sub.2 O.sub.3, about 30.times.10.sup.6 V/cm in the case of Al.sub.2 O.sub.3 and about 70.times.106 V/cm in the case of Si.sub.3 N.sub.4, and thus they are not suited for low-voltage luminescence.
Recently, proposals have been made for use of a thin film mainly composed of PbTiO.sub.3, Pb(Ti.sub.1-x Zr.sub.x)O.sub.3 or like substance having a high dielectric constant as a dielectric layer in an electroluminescent element. These substances are high in .epsilon..sub.i which is over 150, but they are low in E.sub.ib which is on the order of 5.times.10.sup.5 V/cm, so that when using these substances, it is required to greatly increase the film thickness in comparison with the conventional dielectric materials. To guarantee the reliability of the element produced, it is required that the dielectric film have a thickness greater than 15,000 .ANG., for 6,000 .ANG. in thickness of ZnS film. Generally, where said substances are used, the grains in the film tend to grow to cause clouding of the film because of large film thickness and high substrate temperature at the time of formation of the film. In an X-Y matric display using such clouded film, light is emitted even from the non-luminescent segments as the light from the other segments is scattered, resulting in a degraded image quality.
The present inventors had already proposed an EL element using a dielectric film chiefly composed of SrTiO.sub.3, which dielectric film is high in both E.sub.ib and the product of E.sub.ib and .epsilon..sub.i, proof against clouding and suited for low-voltage drive. For instance, there had been obtained an SrTiO.sub.3 dielectric film in which .epsilon..sub.i =140 and E.sub.ib =1.5 MV/cm, the product thereof being greater than that of a BaTiO.sub.3 film (10.ltoreq..epsilon..sub.i .ltoreq.40, E.sub.ib up to 2 MV/cm). Reduction of driving voltage is desirable to improve reliability and production cost of the drive circuits, but no sufficient technical breakthrough has been attained in this regard. In order to increase the luminance of the phosphor layer, this layer is subjected to a heat treatment after formation of the film, but in case a dielectric layer is present beneath said phosphor layer, the dielectric layer also undergoes the heat treatment. Consequently, if the dielectric layer thickness is greater than about 0.5 .mu.m, certain fault is found to take place in the dielectric film, affecting the breakdown strength of the element. Also, the mode of dielectric breakdown tends to become propagating and is unable to self-heal.
The present invention is intended to obtain a dielectric film which is better suited for low-voltage drive and also has higher reliability than said SrTiO.sub.3 dielectric film. It is especially envisaged in this invention to obtain a dielectric film of the type whose dielectric breakdown, if any, is restricted to self heal, keeping free of propagating breakdown which can be a fatal defect for an EL element.